Electronic devices, such as cellular telephones, cameras, and computers, commonly use image sensors to capture images. A typical CMOS imager circuit includes a focal plane array of pixels, and each pixel includes a photosensor, such as a photogate, photoconductor, or photodiode, for accumulating photo-generated charge in a portion of the substrate. Each pixel has a charge storage region, formed on or in the substrate. The charge storage region may be constructed as a floating diffusion region. In some imager circuits, each pixel may include at least one electronic device, such as a transistor, for transferring charge from the photosensor to the storage region, a second device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference, and a third device for amplification of the signal to a readout line.
In a CMOS imager with integrating pixels or pixels that operate in integrating mode, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing a reset level and pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
An important performance characteristic of any imager is its dynamic range. A high dynamic range is desirable in applications for sensing low light signals and capturing images with large variations in luminance or brightness. In particular, the dynamic range of an imager can be defined as the ratio of the minimum luminance the imager detects under saturation to the luminance the imager detects at a signal-to-noise ratio (SNR) equal to one. Examples of high dynamic range scenes include an indoor room with outdoor window, outdoor mixed shadow and bright sunshine, night time scenes combining artificial lighting and shadows, and in an automotive context, an automobile entering or about to leave a tunnel or shadowed area on a bright day.
Intrascene dynamic range refers to the range of incident signals that can be accommodated by an imager in a single frame of image data. In conventional dual conversion gain sensors, a fixed conversion gain is applied to the entire image frame. However, scenes to be imaged often include both brighter and darker portions across any given row of image pixels. Performing image capture operations using conventional image sensors and image processing techniques that control image pixels in an entire frame to operate in the high gain or low gain mode may cause some image pixels to generate excessively noisy, under-exposed, or saturated image signals.
In other imaging systems, the same scene may be captured twice: once in low gain mode and once in high gain mode. Both images can then be used to reconstruct one high dynamic range image. This approach reduces the frame rate by a factor of two and may also include image artifacts due to the time difference between each image capture.